This invention relates to a phonograph stylus, and more particularly to a tubular beryllium phonograph stylus shank and its method of formation.
In a phonograph record playback system including a cartridge with a stylus assembly, for example, as disclosed in U.S. Pat. No. 3,077,522 the magnet-stylus subassembly principally determines trackability, or the ability of the playback system to cope with the amplitude, velocity and acceleration variations demanded by a modulated record groove. The elements of this subassembly are the diamond tip, the stylus cantilever or shank, and the electromechanical transducer, as shown by way of example in FIG. 1 of the accompanying drawing. Advances in recent years to enhance trackability have been directed toward reducing the effective mass of the subassembly. While the diamond tip, the magnet or transducer element, and the shank connecting the two all contribute to the effective mass of the moving "system", the major efforts of research and development have been to reduce the effective mass of the shank, since it is typically the largest contributor to the total system mass. This work has concentrated on selecting shank materials as well as on forming those materials into advantageous shapes. The ideal stylus shank would have infinite stiffness to prevent its bending, to ensure a precise transmission of motional information from the tip to the transducer. Also, the ideal shank would be without mass, so that no inertial forces could inhibit trackability. Available materials, however, vary significantly in approaching these ideals. In recent years, some exotic materials such as beryllium and boron have been identified as material candidates.
Table 1 below lists several materials that, because of their intrinsic properties, may be considered suitable for fabricating stylus shanks. The modulus of elasticity (a characteristic related to stiffness) and the density (a characteristic related to mass) are shown for each material. Also listed is the modulus-density ratio (often called "specific stiffness"), a basic figure of merit (higher is better) when comparing materials for their suitability as shank material. Beryllium, boron, and diamond are seen to be, at least theoretically, significantly more suited as shank material than an old standby, aluminium.
TABLE I ______________________________________ BASIC PROPERTIES OF MATERIALS RATIO MODULUS (MODULUS/ (STIFFNESS) DENSITY) dynes/cm.sup.2 DENSITY cm.sup.2 /sec.sup.2 MATERIAL .times. 10.sup.12 grams/cm.sup.3 .times. 10.sup.12 ______________________________________ ALUMINUM 0.72 2.70 0.27 BERYLLIUM 2.9 1.85 1.58 BORON 4.4 2.34 1.88 SAPPHIRE 3.3-3.9 3.9-4.1 0.93 DIAMOND 7.4-10.5 3.15-3.5 2.88 ______________________________________
While the modulus-density ratio may seem to be the final determining factor in the selection of a shank material, the individual properties of modulus and density cannot be ignored. A material, for instance, with both exceedingly high modulus and density, yet with a high modulus-density ratio, may still result in a shank with excessive effective mass, if that material cannot be formed into a suitable geometric shape that takes advantage of its high ratio.
Table 2 below is similar to Table 1 in that stiffness, effective mass, and their ratio are tabulated. However, Table 2 tabulates these characteristics for various shank geometries, not shank materials.
TABLE 2 __________________________________________________________________________ SHANK GEOMETRY AND PERFORMANCE FACTORS OUTER WALL LENGTH DIAM- THICK- M.sub.EFF STIFFNESS/ "L" ETER, NESS, STIFFNESS (RELATIVE) M.sub.EFF INCHES INCHES INCHES (RELATIVE) (SEE NOTE) RATIO __________________________________________________________________________ ROD 0.25 0.010 -- 1.0 1.0 1.0 TUBE 0.25 0.014 0.002 3.0 1.0 3.0 TUBE 0.25 0.030 0.00075 15.0 0.88 17.0 __________________________________________________________________________ NOTE: Effective mass contribution of shank assumes shank to be pivoted at one end and driven at other end
The material of all the shanks is considered fixed, as is its length. The 0.010" diameter solid rod of 0.250" length is used as a reference and arbitrarily given the reference value of unity for each of the three parameters. The cross sections are then compared relative to this reference. The 0.014" outside diameter tube with 0.0002" wall thickness has the same effective mass as the reference rod, but is three times stiffer. The 0.030" diameter tube with 0.00075" wall thickness, even with somewhat lower mass than the reference, is 15 times stiffer than the solid rod.
Thus, the ability or inability of a material to be formed into a thin-walled structure greatly changes the advantage seemingly provided by favorable properties as tabulated in Table 1. For example, from Table 1, the modulus-density ratio of diamond is seen to be approximately 11 times that of aluminum. But FIG. 2 shows that the modulus-density ratio of a 0.030" diameter thin-walled tube is 17 times greater than a solid rod of 0.010" diameter. In this case then, a thin-walled tube made of aluminum has more than 50% (17/11=1.55) basic performance advantage over a solid rock of diamond.
As a second example, the modulus-density ratio of beryllium is approximately 5.8 times that of aluminum. However, beryllium has been difficult to shape because of its low ductility. Of the known shanks having some beryllium, all but one involve vacuum deposited beryllium, as in U.S. Pat. No. 3,961,797. The one exception is the shank of U.S. Pat. No. 3,909,008, taught at column 4, lines 50-57, to be rolled from beryllium into a tubular form with a longitudinal slit or gap. While this last shank is said to have advantages, the slit appears to weaken the shank by reducing its stiffness, and appears to cause great manufacturing difficulty.